Omnidirectional antennas can detect and generate signals equally well in all directions. Even when a location of a remote communication device is known, however, transmissions to the remote device via an omni-directional antenna can cause energy to be transmitted in wrong directions as well as the right direction. Similarly, when receiving transmissions from a remote device via an omnidirectional antenna, signals from the remote device may be received, as well as interfering signals from devices in different locations around the omnidirectional antenna. The amount of interfering signals may be high, especially if transmitting devices also use omnidirectional antennas.
A switched beam antenna may transmit or receive signals using one of a number of predetermined beams. The antenna may use any one of the predetermined beams that is expected to provide the strongest signal between the antenna and an intended communication device. Thus, a switched beam antenna may provide less interference than an omnidirectional antenna because signals are not transmitted and received in all directions, but are transmitted and received, more or less, in a direction of interest. Further, because transmission and reception are directed in a particular direction, the range of transmission and reception may be increased in the particular direction.
Unlike switched beam antennas, adaptive array antennas do not have predetermined beams. An adaptive array, or electrically steerable, antenna permits a gain pattern to be dynamically modified. Adaptive array antennas include a number of array elements attached to separate antennas. Each array element may transmit or receive a radio frequency (RF) signal simultaneously. Given different gains and/or phases applied to each array element (weights), arbitrary constructive and destructive propagation patterns can effectively be created, thereby causing antenna patterns of gains and nulls.
Typically, adaptive array antennas are used to steer a beam in a previously known direction. This is a difficult task in a wireless network, such as an ad hoc network in which all nodes are moving and signals may be received from any direction at any time. Therefore, there is a need for an adaptive array antenna that can perform dynamic beamforming on a signal arriving from any direction.